1. Field of the Invention
The present invention relates to a direct liquid fuel cell power generating device, and more particularly to a direct liquid fuel cell power generating device using a methanol aqueous solution, and a method of controlling the same.
2. Description of the Related Art
A high performance secondary battery and fuel cell are highly expected as a power source for a portable electronic appliance supporting the information technology society, or as key element in an electric vehicle and a power storage system to solve the problems of air pollution and global warming In particular, as application to the electric vehicle, PEM using hydrogen and oxygen as fuel (PEFC, solid electrolyte fuel cell) has been thought to be hopeful. It is regarded to be suited to an electric vehicle from two viewpoints, that is, a high power is obtained by using clean energy or hydrogen and oxygen, and the power can be restored by refilling with fuel. It is, however, a fatal defect of fuel cell that the power is lowered in the case of a large load current change, and power generation at the time of quick acceleration is difficult. On the other hand, there is already an electric vehicle powered by a lithium ion secondary battery only, but it is considered difficult to apply the lithium ion secondary battery only in the electric vehicle owing to the safety problems or failure of power recovery even by refilling of an electrolyte solution. In this background, the hybrid battery (combination battery) having the features of both the lithium ion secondary battery and fuel cell is becoming important from the viewpoint of application to the electric vehicle. Further, to solve the problem of reduction of fuel volume used in PEM, it has been studied to use compressed hydrogen (250 atm), liquid hydrogen, hydrogen occluded alloy as fuel, or the like. In such a circumstance, the direct methanol fuel cell (DMEC) for generating power by taking out proton directly from methanol is attracting attention from this viewpoint in spite of the defect of smaller power as compared with PEM. By the feature of smaller fuel volume, the direct methanol fuel cell is considered to be applied to portable electronic appliances, and it is highly expected to be applied to multiple fields.
FIG. 15 is a schematic view of a conventional standard direct methanol fuel cell power generating device (Journal of Power Sources, 83, 204, 1999). An electromotive force section of a direct methanol fuel cell comprises an anode electrode including an anode substrate 3 and an anode catalyst layer 2, a cathode electrode including a cathode substrate 5 and a cathode catalyst layer 4, and an electrolyte membrane 1 disposed between the anode electrode and the cathode electrode. As shown in FIG. 16, an anode passage plate 150 has formed therein an anode passage 151 as a methanol channel including a methanol feed port 152 and a methanol discharge port 153. In FIG. 15, a methanol aqueous solution container 16 containing a methanol aqueous solution is connected to the methanol feed port 152 by way of a pump 19. A cathode passage plate 8 has formed therein a cathode passage 11 as a gas channel including an oxidizer feed port 14 and an oxidizer discharge port 15. Oxidizer feed means for feeding an oxidizer such as air is connected to the oxidizer feed port 14.
The electrolyte membrane is, for example, a Nafion membrane having a high proton conductivity. On the other hand, as a catalyst for use in an anode catalyst layer, for example, PtRu of low poisoning is used, or as a catalyst for used in a cathode catalyst layer, for example, Pt is used.
In such a direct methanol fuel cell power generating device, the power is generated in the principle of feeding a methanol aqueous solution to the anode catalyst layer, generating proton by catalytic reaction, and allowing the generated proton to pass through the electrolyte membrane to react with an oxygen supplied in the cathode catalyst layer on the catalyst.
To enhance the power of the direct methanol fuel cell, it is required to maintain a high electromotive force up to a high load current. First, to obtain a high load current, it is needed to increase the methanol volume per unit time to be supplied to the anode catalyst layer. However, the methanol aqueous solution permeates through the Nafion membrane, and a methanol aqueous solution which has not been spent for reaction in the anode catalyst layer reaches the cathode catalyst layer, and induces a similar reaction With the anode catalyst layer In the cathode catalyst layer, thereby producing a counterelectromotive force. This is the so-called crossover overvoltage, which leads to a problem of decrease of the electromotive force in the direct methanol fuel cell. This crossover overvoltage is more serious when the methanol concentration is higher, and when a methanol aqueous solution of 5 M or more is supplied to the anode catalyst layer, the power drops significantly Further, from the viewpoint of suppressing the deterioration of the electrolyte membrane such as a Nafion a membrane, it is preferred to keep the concentration of the methanol aqueous solution at 5 M or less. Hence, to operate the direct methanol fuel cell, it is preferred to keep the concentration of the methanol aqueous solution used as fuel at 5 M or less.
In another method of lowering the crossover overvoltage, all methanol supplied to the anode catalyst layer is spent in the anode catalyst layer, and is not passed into the cathode catalyst layer. That is, the catalyst activity in the anode catalyst layer is improved, or the catalyst carrying amount is increased, but such a method cannot be realized in the existing catalysts. It has been also attempted to develop a catalyst member not allowing the methanol aqueous solution which has not been spent in the anode catalyst layer to pass through the cathode catalyst layer, but actually such electrolyte membranes are mostly poor in the proton conductivity, and the power is lowered to the contrary. Hence, to lower the crossover overvoltage, it is effective to lower the methanol concentration in the methanol aqueous solution used as fuel. However, when using fuel of low concentration, a large fuel container is needed, which is contradictory to the feature of the direct methanol fuel cell.
Thus, although it is preferred to use a methanol aqueous solution at concentration of 5 M or less, if a dilute methanol aqueous solution of about 1 M is used as fuel, not only the volume of the methanol aqueous solution container is increased, but also methanol shortage is likely to occur in the reaction in the battery inside. Therefore, it is necessary to feed the methanol aqueous solution promptly from the methanol aqueous solution container.
In such operation, although the crossover overvoltage can be lowered and the battery power is increased, the pump power for supplying the methanol aqueous solution increases, and hence the power of the entire power generating device is lowered to the contrary.
As explained herein, from the viewpoint of fuel supply, it is preferred to feed a concentrated methanol aqueous solution at a small flow rate, but from the viewpoint of power, to the contrary, it is preferred to feed a dilute methanol aqueous solution at a large flow rate. That is, to decrease the fuel volume and obtain a high power, it is required to supply the methanol aqueous solution at the optimum concentration at an optimum flow rate.
As shown in FIG. 15 and FIG. 16, an ordinary direct methanol fuel cell is designed to supply a methanol aqueous solution to the anode passage plate 7 by means of a pump. The methanol aqueous solution supplied from the pump 19 flows in the portion of a groove 151 of the passage plate through an inlet 152 of the passage plate. The convex portion of the passage plate contacts with the anode substrate 3 such as an anode carbon paper, and the methanol aqueous solution flowing in the anode passage 10 permeates into the anode substrate 3, so that methanol is supplied into the anode catalyst layer 2.
However, all methanol aqueous solution flowing in the anode passage 10 does not permeate into the anode substrate 3, and is partly discharged from the methanol discharge port 153 of the passage plate 7. Accordingly, the utilization efficiency of the methanol aqueous solution in the container is generally low.
It has been attempted to modify the structure of the passage plate in order to enhance the efficiency, but the utilization efficiency has not been dramatically improved yet at the present. Alternatively, as shown in FIG. 17, it may be considered to design a mechanism of returning the methanol aqueous solution discharged from the discharge port 13 of the anode passage plate 7 back to the container. However, since methanol and water are spent equally in the anode catalyst layer, if the methanol aqueous solution discharged from the anode passage plate is returned to the container 16, the concentration of the methanol aqueous solution in the container is gradually lowered. As a result, methanol shortage occurs in the electromotive force section, and the electromotive force is decreased suddenly. Therefore, in the case of the liquid feed method in FIG. 17, a concentrated methanol aqueous solution of nearly 5 M must be used as the fuel. It is hence possible to suppress sudden methanol shortage in the battery and generate power for a long period of time, which will be effective liquid feeding means.
On the other hand, in reaction in the anode electrode, carbon dioxide is produced. As shown in FIG. 17, in a mechanism of returning an excess portion of the methanol aqueous solution into the methanol aqueous solution container by connecting the discharge port 13 of the anode passage and the methanol aqueous solution container 16, the carbon dioxide produced in the anode electrode is accumulated in the methanol aqueous solution container, and the internal pressure in the methanol aqueous solution container is elevated. Also, as shown in FIG. 15, even when an excess portion of the methanol aqueous solution is collected in another container without returning to the methanol aqueous solution container, if the container has a finite volume, the internal pressure in the container gradually climbs up. Anyway, the discharge pressure of the feed pump must be gradually raised, which results in drop of the entire power of the direct methanol fuel cell power generating device. Further, when carbon dioxide is collected at the anode electrode side, the effective catalyst surface area of the anode catalyst 2 is decreased, which may also result in an power drop. To avoid such inconvenience, the carbon dioxide produced at the anode electrode must be discharged to outside. However, since the methanol used as the fuel has a low boiling point, about 62° C., and it is easily gasified and mixed with carbon dioxide. Besides, since methanol is harmful for human health, 260 ppm or more must not be released to the atmosphere, Therefore, methanol gas and carbon dioxide must be separated and discharged separately. By the present technology, it is not easy to separate methanol gas and carbon dioxide. On the other hand, to decrease the evaporating amount of methanol gas, it has been considered to operate the fuel cell power generating device at low temperature. However, as compared with the operating temperature of 70 to 80° C. for obtaining the maximum power of the direct methanol cell, the power drops to not more than one-third at 30 degrees. It is hence impossible to generate power from the direct methanol fuel cell at low temperature.
Moreover, the methanol diffused in the anode catalyst layer 2 should be ideally spent entirely in reaction, but in the present power generation, byproducts are produced. Byproducts include formaldehyde and formic acid, and they are harmful for the environment and human health, and atmospheric emission is controlled. In particular, formaldehyde is toxic, and its atmospheric emission is strictly restricted to 0.5 μm or less. If these substances are mixed in the carbon dioxide, they may be discharged from the anode electrode side, and also same as methanol, they are high in solubility in water, and may be mixed in water produced in the cathode electrode, and the possibility of discharge from the cathode electrode side seems higher. Therefore, since the water produced in the cathode electrode contains methanol, formaldehyde, and formic acid, the water produced in the cathode electrode cannot be directly discharged outside. Moreover, since formaldehyde exists as gas at room temperature, it maybe discharged as gas from the cathode electrode. Therefore, when an excess oxidizer discharged from the cathode electrode is collected, formaldehyde may be contained, so that the gas collected from the cathode electrode by way of the oxidizer outlet 14 cannot be directly discharged outside.
In a method of separating carbon dioxide and methanol gas, for example, a mixed gas of carbon dioxide and methanol gas, is blown into an absorption pipe tilled with, as an adsorbent, an inorganic matter such as calcium chloride or magnesium chloride, activated carbon, or an organic matter for absorbing alcohol such as polyacrylic amide gel. This absorption pipe operates on the principle that these adsorbents adsorb methanol which is high in polarity, but does not adsorb carbon dioxide which is not polar. This adsorption pipe tray be also used for separating the gas discharged from the anode electrode of the direct methanol fuel cell power generating device, but the adsorbent must be replaced periodically. Further, since the operating temperature of 70 to 80° C. of the direct methanol fuel cell power generating device exceeds the boiling point of methanol, a massive methanol gas is discharged from the anode electrode. As a result, when such methanol gas is absorbed inside the absorption pipe, the fuel utilization efficiency is extremely lowered. Further, if an organic matter such as nitrophenyl hydrazine is used as the adsorbent in the absorption pipe, although the oxidizer generated from the cathode electrode and formaldehyde can be separated, water is generated from the cathode electrode, and therefore the absorption pipe may be filled with water. It is hence necessary to separate the liquid components first from the substances discharged from the cathode electrode, and then separate by using the absorption pipe, and the piping in the direct methanol fuel cell power generating device is complicated.